docs: Fix iptables rule to access Tuskar UI
docs: Fix iptables rule to access Tuskar UI

Using the iptables append action (-A) always adds the rule after the REJECT
rule and all the packets are dropped.
Changing the command by insert action (-I) adds the rule at the top of the
INPUT list.

Change-Id: I56a145c1d66d95ecd4d5d5c496d1d33de56fb2d6

dpkshetty/instack-undercloud,eprasad/instack-undercloud,dmsimard/instack-undercloud,dmsimard/instack-undercloud,rdo-management/instack-undercloud,bcrochet/instack-undercloud,eprasad/instack-undercloud,bcrochet/instack-undercloud,dpkshetty/instack-undercloud,rdo-management/instack-undercloud